This section introduces aspects that may facilitate better understanding of the present disclosure. Accordingly, the statements of this section are to be read in this light and are not to be understood as admissions about what is in the prior art or what is not in the prior art.
The fast uptake of the Third Generation Partnership Project (3GPP)-Long Term Evolution (LTE) in different regions of the world shows both that demand for wireless broadband data is increasing, and that LTE is a successful platform to meet that demand. The existing and new spectrum licensed for exclusive use by International Mobile Telecommunications (IMT) technologies will remain fundamental for providing seamless coverage, achieving the highest spectral efficiency, and ensuring the highest reliability of cellular networks through deliberate planning and deployment of high-quality network equipment and devices.
In order to meet the ever increasing data traffic demand of users, particularly in concentrated high traffic buildings or hot spots, more mobile broadband bandwidth will be needed. Given a large amount of spectrum available in unlicensed bands around the globe, the unlicensed spectrum is more and more considered by cellular operators as complementary means to augment their service provisioning. While the unlicensed spectrum cannot match the qualities of the licensed regime, solutions that allow efficient use of it as a complement to licensed deployment have a potential to bring a great value to 3GPP operators, and ultimately to the 3GPP industry as a whole. This type of solutions would enable operators and vendors to leverage the existing or planned investments in LTE/Evolved Package Core (EPC) hardware in radio and core networks.
It has been agreed to study Licensed-Assisted Access (LAA) technologies in the 3GPP at RP-141664. This LAA framework builds on carrier aggregation solutions introduced in LTE Release-10 to access the additional bandwidth in the unlicensed spectrum. As illustrated in FIG. 1, an LTE network can configure a user equipment (UE) to aggregate additional secondary cells (S Cells) which are using frequency carriers in the unlicensed spectrum. The primary cell (PCell) maintains exchange of essential control messages and also offers an always-available robust spectrum, i.e. the licensed spectrum, for real-time and high-value traffic. Via the high-quality and robust licensed spectrum, the PCell may also provide mobility handling and management for the UE. The aggregated SCells in the unlicensed spectrum, when available, can be utilized as a bandwidth booster to serve, e.g. the best effort traffic. The LAA S Cell may operate in a downlink (DL)-only mode or operate with both uplink (UL) and DL traffic.
In 3GPP LTE systems, retransmissions of missing or erroneous data blocks are handled primarily by a Hybrid Automatic Repeat reQuest (HARQ) mechanism at the Medium Access Control (MAC) layer, complemented by the retransmission functionality of the Radio Link Control (RLC) protocol. With all resources in control, a base station at the network side may allocate resources for Acknowledgement/Negative Acknowledgement (ACK/NACK) feedbacks and retransmissions in a predefined way, so that the timing relationship for a new transmission and ACK/NACK, and for ACK/NACK and a retransmission is preconfigured by the 3GPP standard, e.g. in 3GPP TS 36. 213 V 11.7.0. For example, in a frequency division duplex (FDD) system, the timing is fixed to be a 4 ms offset, while in a time division duplex (TDD) system, the timing is fixed according to different UL/DL configurations. Furthermore, a maximum retransmission number is usually configured by the base station for the UE, e.g. by setting a parameter maxHARQ-Tx in the information element MAC-MainConfig as defined in 3GPP TS 36. 213 V 11.7.0, so that the UL HARQ behavior of the UE may be in control.
For a wireless local area network (WLAN) system operating in the unlicensed spectrum, the usage of HARQ is abandoned, mainly due to uncertainty of resource availability. Instead, the WLAN system only relies on an Automatic Repeat reQuest (ARQ) scheme defined at the logical link control (LLC) layer. In other words, the existing HARQ mechanism may not be applicable to unlicensed carriers of the unlicensed spectrum.
One main concern to apply the existing HARQ mechanism to unlicensed carriers directly is the uncertainty of resource availability on the unlicensed carriers. In a scenario where the collision probability on a specific unlicensed carrier is high, HARQ transmissions would cause a latency increase, which may be even longer than the t-Reordering timer defined for ARQ at the RLC layer. As defined in 3GPP TS 36.322 V12.2.0, the t-Reordering timer is used by the receiving side of an Acknowledged Mode (AM) RLC entity and receiving Unacknowledged Mode (UM) RLC entity in order to detect loss of RLC Packet Data Units (PDUs) at the lower layer, i.e., it is used to hold back the retransmission request until the receiving side is sure that the data unit is indeed lost. When the t-Reordering timer expires, a retransmission at the RLC layer will be started. In this case, it is useless to keep the old copy of the data unit in the HARQ buffer anymore.
Currently, there is not any differentiation in terms of the HARQ mechanism in the 3GPP standard with regard to different carriers.